The present invention relates to a method of making agglomerated cemented carbide powder mixtures with high binder phase content.
Cemented carbide alloys are made of hard constituents based on carbides in a binder phase essentially based on Co and/or Ni. A binder phase content of from about 5 to about 15 wt-% is generally found in tools for metal machining and rock drilling and many wear parts. A low binder phase content results in an alloy with high wear resistance and low toughness whereas a high binder phase content gives an alloy with higher toughness and lower wear resistance. In applications demanding extremely high wear resistance, a binder phase content of less than about 5 wt-% can be used. Components with such low binder phase content have to be mounted under prestressed conditions. Examples of are seal rings where a further advantage of the low binder phase content is the increased corrosion resistance. On the other hand in applications where high toughness is indispensable yet good wear resistance is needed, binder phase contents of from about 20 to about 30 wt-% are used. A typical example is rolls for hot rolling. A further advantage of the high binder phase content in such rolls is that they can easily be reground when worn.
Cemented carbide bodies are made by powder metallurgical methods comprising wet milling a powder mixture containing powders forming the hard constituents and binder phase as well as pressing agents and other additives often of proprietary character, drying the milled mixture to a powder with good flow properties, pressing the dried powder to bodies of desired shape and finally sintering.
The intensive milling operation is performed in mills of different sizes using cemented carbide milling bodies. Milling is considered necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. The milling time is in the order of several hours up to days. The milling operation produces a slurry which is suitable for subsequent spray drying. Successful spray drying depends strongly on the slurry properties. The viscosity of the slurry has to be optimised, shear thickening has to be avoided and sedimentation has to be minimised. Sedimentation will result in inferior properties of spraydried powders and may cause severe flowability problems. The current technology of intensive milling during extended period of times usually produces a very fine grained powder suspension in which little or no sedimentation takes place. As a result of the spray drying process spherical agglomerates of about 0.1 mm diameter are obtained held together by the pressing agent. This is true for cemented carbide compositions with a medium to low binder phase content. However, for binder phase contents of from about 20 to about 30 wt-% for some at present unknown reason the agglomerates formed have inferior properties which results in a ready to press powder with very bad flow properties not useful for automated production. Instead more manual methods have to be applied in order to ensure a satisfactory sintered product.